b) Có 7 cây trồng thành 6 hàng mỗi hành 3 cây

a ) Có 10 cây trồng thành 5 hàng mỗi hàng 4 cây .

\(\dfrac{4}{2.4}+\dfrac{4}{4.6}+\dfrac{4}{6.8}+...+\dfrac{4}{16.18}+\dfrac{4}{18.20}\)

= \(\dfrac{4}{2}\left(\dfrac{2}{2.4}+\dfrac{2}{4.6}+\dfrac{2}{6.8}+...+\dfrac{2}{16.18}+\dfrac{2}{18.20}\right)\)

= \(\dfrac{4}{2}\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{8}+...+\dfrac{1}{16}-\dfrac{1}{18}+\dfrac{1}{18}-\dfrac{1}{20}\right)\)

= \(\dfrac{4}{2}\left(\dfrac{1}{2}-\dfrac{1}{20}\right)\)

= \(\dfrac{4}{2}.\dfrac{9}{20}\)

= \(\dfrac{9}{10}\)

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Lỗi:

Thiếu động từ

Sửa Lại :

We are going to visit Hue Citadel in our summer vaction .

\(\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+\dfrac{2}{9.11}+\dfrac{2}{11.13}+\dfrac{2}{13.15}\)

= \(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{15}\)

= \(\dfrac{1}{3}-\dfrac{1}{15}\)

= \(\dfrac{4}{15}\)

Cho khối nón có bán kính đáy r = 2, chiều cao h = 3 . Thể tích của khối nón là:

A.  4 π 3

B.  2 π 3 3

C.  4 π 3

D.  4 π 3 3

Ta thấy :

\(\dfrac{1}{2^2}< \dfrac{1}{1.2}\)

\(\dfrac{1}{3^2}< \dfrac{1}{2.3}\)

................

\(\dfrac{1}{2^n}< \dfrac{1}{n.\left(n-1\right)}\)

\(\)- > \(\dfrac{1}{2^2}+\dfrac{1}{3^2}+\dfrac{1}{4^2}+...+\dfrac{1}{2^n}< \dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{\left(n-1\right).n}\)= \(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{n-1}-\dfrac{1}{n}\)= \(1-\dfrac{1}{n}< 1\left(ĐPCM\right)\)